1. Field of the Invention
The present invention relates to microelectromechanical actuator structures, and more particularly to electrostatically activated micromachined actuator structures.
2. Description of the Related Art
Advances in thin film technology have enabled the development of sophisticated integrated circuits. Such semiconductor technology has also been leveraged to create Micro Electro Mechanical System (MEMS) structures. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices.
Flexible composite electrostatic actuators typically include a flexible composite fabricated from flexible electrode and insulators. The flexible composite is attached to and can deflect under an electrostatic force toward a substrate including a fixed electrode. An insulator is disposed between the flexible composite and the substrate to avoid shorting of the flexible electrode to the fixed electrode. By applying a voltage between the flexible electrode and the fixed electrode, the flexible composite is pulled to the substrate by electrostatic attraction. Without voltage, typically stress in the flexible composite curls the flexible composite away from the substrate. Applications for flexible composite actuators include gas or fluid valves, optical shutters, radio frequency phase shifters, choppers for infrared detectors, microactuators, electrical switches, and variable radio frequency capacitors
A conventional actuator of U.S. Pat. No. 6,236,491 is shown in FIG. 1. The actuator therein includes a fixed composite 130 and a flexible composite 50. The fixed composite 130 includes a substrate 10, a fixed electrode 20, and a substrate insulator 30. A flexible composite 50 including a flexible electrode 40 overlies the fixed composite 130, and includes a fixed portion 70, a medial portion 80, and a distal portion 100. A fixed portion 70 is substantially affixed to the underlying substrate 10 or intermediate layers. A medial portion 80 extends from the fixed portion 70 and is held in position without the application of electrostatic force, thereby defining an air gap 120 between the underlying planar surface and the medial portion 80.
Both the medial portion 80 and the distal portion 100 are released from the underlying fixed composite 130 upon completion of the actuator. The distal portion 100 is free to move in operation, curling away and altering the separation from the underlying planar surface. Once the flexible composite 50 bends the medial portion 80 can curl toward, curl away, or remain at a constant separation from the underlying planar surface.
In cross section, the flexible composite 50 can include multiple layers including at least one electrode layer 40 and can include a biasing layer to mechanically reinforce a section of the flexible composite toward the fixed portion 70. The number of layers, thickness of layers, arrangement of layers, and choice of materials used may be selected to cause the flexible composite to curl toward, curl away, or remain parallel to the underlying microelectronic substrate electrode.
The flexible composite 50 typically include a polymer film 60, a flexible electrode 40, and another polymer film 62. Different thermal coefficients of expansion between the layers of the flexible composite 50 mechanically bias the medial portion 80 and distal portion 100 to curl away from the underlying surface 32 after removal of a release layer 34 used in fabrication of the structure. The distal portion 100 can curl with either a variable or constant radius of curvature.
Because the medial portion is constructed similarly to the distal portion, the differential thermal expansion coefficients between the electrode 40 and polymer film(s), tend to curl the medial portion. However, additional layers of polymer film, metals, or other materials may optionally be applied over the second layer of polymer film to serve as a biasing control structure to counteract the tendency to curl and hold the medial portion in position once the release layer has been removed. Alternatively, materials may be applied with intrinsic stresses to enhance the tendency to curl and increase the distance between the flexible composite and the substrate surface.
Despite the sophistication of conventional actuators, a number of problems affect the reliability and performance of the actuators. These problems detailed below are addressed in the various embodiments of the present invention.